Thermal fuse having telescopically received contact members



Oct. 25, 1966 G. F. EBENSTEINER 3,281,559

THERMAL FUSE HAVING TELESCOPICALLY RECEIVED CONTACT MEMBERS Filed May21, 1964 United States Patent THERMAL FUSE HAVING TELESCOPICALLYRECEIVED CONTACT MEMBERS Gerald F. Ebensteiner, Minneapolis, Minn.,assignor, by

mesne assignments, to United-Carr Incorporated, Boston, Mass., acorporation of Delaware Filed May 21, 1964, Ser. No. 369,087

6 Claims. (Cl. 200-142) This invention relates generally to circuitbreaking devices, and pertains more particularly to a thermal fuse whichinterrupts or opens an electric circuit when a predetermined ambienttemperature is reached.

There are a number of situations in which it is necessary or at leastdesirable that a circuit he opened when an excessive or abnormaltemperature is experienced. This is for the purpose of protectingtemperature-vulnerable equipment from damage in the region where theprotective device is installed.

It will be apparent that some uses for thermal devices of the envisagedtype require that a miniaturized thermal fuse be employed because spacesimply does not exist for a thermal fuse of any appreciable size.Accordingly, one object of the present invention is to provide a thermalfuse that lends itself readily to miniaturization.

The invention has for another object the provision of a thermal fusethat will be quickly responsive to a rise in temperature, therebyobviating the chances of damage occurring from objectionable time lagsor delays.

Another object of the invention is to provide a thermal fuse that can beeasily assembled and which will not require close manufacturingtolerances. In this regard, the invention has for an aim the employmentof telescopically received contact members that can be initially engagedWithout difliculty when the parts are being put together, yet which willseparate easily when the predetermined temperature is reached.

Another object of the invention is to provide a thermal fuse of theforegoing character requiring only a relatively few parts so that it canbe manufactured at a comparatively low cost.

Yet another object of the invention is to provide a thermal fuse thatcan be sealed in order that it will be made moisture-proof and alsogas-tight, thereby allowing it to be utilized where high humidity is aptto be experienced or in situations where explosive conditions are likelyto be encountered.

These and other objects and advantages of my invention will more fullyappear from the following desorption, made in connection with theaccompanying drawing, wherein like reference characters refer to thesame or similar parts throughout the several views and in which:

FIGURE 1 is a longitudinal sectional view of one form my invention mayassume;

FIGURE 2 is a sectional view taken in the direction of line 22 of FIGURE1 in order to show to better advantage the construction of the movablecontact;

FIGURE 3 is a longitudinal sectional view corresponding to FIGURE 1 butshowing a diflerent embodiment;

FIGURE 4 is a longitudinal sectional view of the modified movablecontact before it is placed within the casing shown in FIGURE 3, theview being in the direction of line 4-4 of FIGURE 5, and

FIGURE 5 is an end view taken from the right in FIGURE 4.

Referring now in detail to the embodiment depicted in FIGURES 1 and 2,the thermal fuse there exemplifying my invention has been designatedgenerally by the reference numeral 10. The fuse comprises a metalliccylindrical casing or shell 12 having a side wall 14, a closed end wall16 and an inturned flange or lip 18. A

shoulder 20 is formed internally of the casing or shell 12. Interposedbetween the flange 18 and the shoulder 20 is a metallic ring or rim 22that is held captive between the flange and shoulder. The ring 22 ispart of a glass seal assembly which additionally includes an annularmember 24, such as of glass, ceramic or a suitable plas tic, onto whichis shrunk the ring 22. The glass or porcelain annular member 24 hasextending therethrough an electric terminal or lead 26. A suitablesealant or potting compound is applied at 28; this compound can be ofthe epoxy resin type.

Whereas the electric terminal or lead 26 is located at one end of thecasing 12, a second terminal or lead 30 is employed at the closed end ofthe casing. The terminal 30 is soldered, brazed or welded directly tothe end wall 16 in the illustrated embodiment, the soldered attachmentbeing designated by the reference numeral 32.

A fixed pin contact 34 extends axially into the casing 12, being in thisinstance a continuation of the terminal 26. It will be understood,however, that the pin 34 can be either larger or smaller in diameterthan the terminal 26 depending upon design requirements. While thecontact 34 is fixedly located within the casing 12 by reason of the factthat the terminal or lead 26 is held tight by the annular member 24, asecond contact 36 in the form of an appropriately bent metallic stripcoacts with the fixed contact 34, being free to move relative to saidcontact 34. The configuration of the movable contact 36 as shown inFIGURE 2 is of appreciable importance as far as fabricating the fuse 10is concerned. From FIGURE 2, it will be discerned that the movablecontact 36 includes a reversely bent or U-shaped portion indicatedgenerally by the reference numeral 38 having a semicircular bight 40 andsides 42, the sides 42 being somewhat undulated and bulging in eachinstance at 44 so as to accommodate therebetween the pin contact 34. Thebulges at 44 merge into substantially parallel shanks 46 and the shanks46 are in turn integrally connected to arcua-te arms 48 extending aboutthe interior of the shell 12 toward the bight 40.

As further shown in FIGURE 2, the shanks 46 are separated somewhat fromeach other due to the fact that the diameter of the pin contact 34 issomewhat greater than the normal or natural diameter existing between0pposite points on the bulges 44. In this way, a firm frictionalengagement is effected between the pin contact 34 and the movablecontact 36. Stated somewhat differently, the movable contact 36 has itsreversely bent portion 38 in what can be considered to be an encirclingrelationship with the pin contact 34. While good fractional engagementis derived by reason of the particular configuration of the contact 36,nonetheless facile separation can be achieved in a manner presently tobe described. Also, it will be appreciated that the arcuate arms 48,through the agency of their inherent resiliency, frictionally bear onthe interior of the shell 12. Inasmuch as the strip 36 functions as anelectrical contact, it follows that it should be of metal having goodelectrical conductivity, as well as good resiliency, a berylliumcopperalloy being a satisfactory material, although other metals may beutilized.

Attention is now directed to the presence of a bushing 50 ofelectrically insulating material. The bushing 5'0 has a bore 52 which isslightly greater in diameter than the diameter of the pin contact 34,thereby allowing the bushing to move freely along the pin contact whenrequired to do so as will hereinafter be explained. The bushing 50 has aflange 54 thereon which is formed with a diameter only slightly lessthan that of the interior of the casing 12. Thus, whereas the bushing isfree to move along the pin contact 34, it is also free to move relativeto the casing 12. Still further, the relative diameters of the bore 52and the flange 54 are such that the left end of the bushing as shown inFIGURE 1 will bear or press against the reversely bent portion 38 andalso the arcuate arms 48 of the movable contact 36, thereby avoiding anytendency for the contact to bind or cock within the casing 12.

In order to bias or urge the movable contact 36 to the left, which is ina direction to cause separation of its reversely bent portion 38 fromits encircling engagement with the pin contact 34, a coil spring 56 ispositioned between the annular member 24 and the flange 54 belonging tothe bushing 50. Thus, the coil spring, by reason of its compressedstate, continually acts in a direction against the movable contact 36,doing so through the medium of the bushing 50, so as always to tend toseparate the contact 36 from the contact 34.

A washer 58 is placed next to the movable contact 36 and transmits theforce of the coil spring 56 against a temperature sensitive pellet orelement 60. The precise composition of the temperature sensitive pellet60 will depend upon the temperature at which the contacts 36, 34 are toseparate. In other words, the pellet 60 will normally resist or preventany such separation but when it becomes sufficiently soft or molten dueto heat being transmitted through the casing 12 thereinto, it will allowthe spring 56 to disengage the contact 36 from the contact 34. Varioustemperature sensitive pellets are currently available, and the specificpellet 60 would be selected in accordance with the particulartemperature at which it should soften in order to safeguard theequipment it is intended to protect.

It will be apparent that my thermal fuse can be easily assembled. theinturned flange 18 thereon. This permits the inser tion of the pellet 60followed by the placement of the washer 58 thereagainst. The contact 36can then be inserted and the bushing 50 can be placed directly againstthe contact 36 as shown or a washer similar to the washer 58 can beadded. By then introducing the spring 56, all that remains to be addedis the assembly or unit comprised ofthe ring 22, the annular member 24and the terminal 26 which has integral therewith in the illustratedinstance the pin contact 34. This assembly is pressed against the coilspring 56 with sufficient force to cause it to be compressedsufficiently so that the right end of the casing 12 can be rolled orformed into the flange 18. With the ring 22 abutting against theshoulder 20, it will be recognized and appreciated that a firm retentionof the ring 22 is realized by means of the inturned flange 18. In thisway, the pin contact 34 is fixedly anchored and projects axiallyinwardly relative to the casing 12. The compound 28 can be applied as anadded sealing precaution.

It should be particularly noted that the contact pin 34 need not be ofany precise length. This is so by virtue of the telescoping engagementthereof with the movable contact 36. If the left end of the 'pin contact34 resides farther to the right than it appears in FIGURE 1, it will notmake any difference because there will still be an adequate engagementbetween the pin and the reversely bent portion '38 of the contact 36. Inother words, the pin contact 34 may vary in length almost as much as thewidth of the strip constituting the movable contact 36. It will also beobserved that only one coil spring 56 isutilized in a construction suchas that which has been described. Consequently, only a few parts arerequired in order to provide a thermal fuse constructed in accordancewith the teachings of this invention.

The operation of my thermal fuse should be obvious from what has beenset forth. With the parts assembled as shown in FIGURE 1, when the fuse10 is subjected to a sufficiently high ambient temperature, the pelletor element 60 begins to melt and it quickly changes from its solid stateto a semi-liquid state. This allows the com- Initially, the casing 12does not have.

pressed spring 56 to move the contact 36 to the left. Any displacementof the material constituting the element 60 can move through the centralopening in the washer 58 and is then free to pass through the opening inthe reversely bent portion 38 formed by the semicircular bight 40 intothe void provided by the configuration of the contact 36. Consequently,there is no blocking of the molten material after the pellet 60 hasbecome soft due to sufiicient heat being conducted thereinto. Therefore,because of the construction that has been described, it is not necessaryto use a pellet or element 60 that will vaporize, instead it being onlynecessary to employ a pellet of the type that will become soft andflowable due to sufficient heat.

Directing attention now to FIGURES 3-5, a second embodiment of theinvention has there been illustrated, being differentiated from thefirst embodiment by adding the suffix a to the reference numeral 10.Actually, the embodiment it of FIGUREl is regarded to be superior to themodified arrangement 10a because of the simplicity with which thecontact 36 can be fabricated. The movable contact of the thermal fuse1011, such contact having been denoted by the reference numeral 36a,includes an inner tubular portion 62 having a plurality of springfingers 64 which frictionally bear against the pin contact 34. Thesefingers 64 can be readily provided by merely slitting what wasoriginally a true tubular portion, the slits having been indicated bythe reference numeral 66. The movable contact 36a also comprises anouter tubular portion 68 having a plurality of spring fingers 70 formedby means of slits '72. The concentric tubular portions 62 and 68 arejoined by means of an annular base '74. Owing to the differentconfiguration of the contact 36a from the contact 36, a somewhat longercoil spring 56a should be employed than the spring 56 in order to exertthe proper displacement force against the base 74 of the contact 36a.

The manner of assembling the thermal fuse denoted as 10a is identicalwith the procedure utilized in assembling the unit 10. Also, theoperation is generally similar because the pellet or element 60 becomessufficiently soft, the coil spring 5601 is instrumental in urging themovable contact 36a in a direction to cause separation thereof withrespect to the pin contact 34. Normally, however, the spring fingers 64produce a good frictional engagement with the pin contact 34 andlikewise the spring fingers 70 provide a good frictional engagement withthe shell 12. To accomplish this, the fingers 64 have their free endsbent slightly inwardly and the fingers 70 have their free ends bentslightly outwardly. The degree of convergence and divergence is so smallas not to be noticeable in FIGURE 4.

In both embodiments, a circuit normally exists between the terminals 26,30. The electrical path is through the terminal 26, the pin contact 34which is integral therewith, the movable contact 36 (or 36a) and thecasing 12 which has attached thereto the other terminal 30. However,when the two contacts 34, 36 (or 34, 36a) are separated, then thecircuit is interrupted or opened to efiect the desired control madenecessary due to the excessively high temperature.

It will, of course, be understood that various changes may be made inthe form, details, arrangements and proportions of the parts withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What is claimed:

1. A thermal fuse comprising:

(a) a cylindricalcasing;

(b) a pair of electric terminals projecting in opposite directions fromthe ends of said casing;

(c) a pin contact extending axially inwardly from one of said terminals;

(d) a unitary movable contact having a reversely bent portionfrictionally engaging the opposite sides of said pin contact and anarcuate arm integral with each end of said reversely bent portion, saidarms bearing frictionally against the inside of said cylindrical casing;

(e) a coil spring acting against said movable contact to bias saidmovable contact in an axial direction to separate said reversely bentportion from said pin contact, and

(f) a temperature sensitive element normally preventing separation ofsaid reversely bent portion from said pin contact until said temperaturesensitive element has become softened by sufficient heat.

2. A thermal fuse comprising:

(a) a cylindrical casing;

(b) a unitary movable electrical contact including a pair of arcuatearms having a radius of curvature corresponding generally to that ofsaid casing so as to bear frictionally thereagainst and a reversely bentportion integral with said arms and extending generally diametricallyacross the interior of said casing;

(c) a fixed pin contact normally projecting axially into frictionalengagement with said reversely bent portion to establish an electricalpath therebetween;

(d) a coil spring acting upon said movable contact to cause separationthereof from said fixed contact, and

(e) a temperature sensitive element normally resisting movement of saidmovable contact until said temperture sensitive element has beensoftened by heat.

3. A thermal fuse comprising:

(a) a cylindrical casing;

(b) a fixed pin contact;

(c) a movable strip contact,

((1) said strip contact having a generally U-shaped centr-al portionhaving its sides resiliently engaging said pin contact and havingarcuate arms resiliently engaging the interior of said casing;

(e) a bushing member encircling said pin contact provided with an innerdiameter of a size so as to cause it to bear against said U-shapedportion and an outer diameter so as to cause it to bear against saidarcuate arms;

(f) a coil spring engaging said bushing member so as to urge saidbushing member axially within said casing to cause said strip contact toalso move axially to an extent necessary to separate said U-shapedportion from said pin contact, and

(g) a temperature sensitive element normally blocking movement of saidstrip contact until said temperature sensitive element has meltedsufiiciently to allow said strip contact to be moved under the influenceof said spring.

4. A terminal fuse in accordance with claim 3 includ- (a) a firstterminal connected to said fixed pin contact,

and

(b) a second terminal connected to said casing,

(c) in which said casing is electrically conductive and said bushingmember is non-conductive whereby an electrical path is establishedthrough said pin contact, said strip contact and said casing until saidtemperature sensitive element has become sufficiently molten.

5. A thermal fuse comprising:

(a) a cylindrical casing;

(b) a pair of electric terminals projecting in opposite directions fromthe ends of said casing;

(c) a pin contact extending axially inwardly from one of said terminals;

(d) a movable contact having concentric inner and outer tubular portionsand a connecting portion therebetween,

(e) said inner tubular portion having a plurality of spring fingersfrictionally engaging said pin contact and said outer tubular portionhaving a plurality of of spring fingers frictionally engaging theinterior of said casing;

(f) a coil spring acting against said connecting portion to bias saidmovable contact in an axial direction to separate said inner tubularportion from said pin contact, and

(g) a temperature sensitive element normally preventing separation ofsaid inner tubular portion from said pin contact until said temperaturesensitive element has become softened by sufficient heat.

6. A thermal fuse comprising:

(a) a cylindrical casing;

(b) a fixed pin contact projecting into said casing from one end;

(c) a movable contact cooperable with said fixed contact having a firstportion thereof resiliently and frictionally engaging the interior ofsaid casing and a second portion thereof resiliently and frictionallyengaging only the sides of said fixed contact;

(d) spring means urging said movable contact in a direction to causedisengagement of said movable contact from said fixed contact, and

(e) a temperature sensitive pellet adjacent the other end of said casingfor resisting movement of said movable contact until said pellet hasbecome softened by suflicient heat.

References Cited by the Examiner UNITED STATES PATENTS 1,691,705 11/1928 Cook 200-123 2,037,544 4/1936 Seaman 200-166 2,180,711 11/ 1939Lehmann 200-142 X 2,750,469 6/1956 Baker 200-114 X 2,810,893 10/1957Nijland 200-166 X 2,934,628 4/1960 Massar et al. 200-117 X 2,955,17910/1960 Milton et al. 200-142 2,968,787 1/1961 Wooton 339-213 3,169,8172/1965 McKee 339-256 3,180,958 4/1965 Merrill 200-142 BERNARD A.GILHEANY, Primary Examiner. H. B. GILSON, Assistant Examiner.

2. A THERMAL FUSE COMPRISING: (A) A CYLINDRICAL CASING; (B) A UNITARY MOVABLE ELECTRICAL CONTACT INCLUDING A PAIR OF ARCUATE ARMS HAVING A RADIUS OF CURVATURE CORRESPONDING GENERALLY TO THAT OF SAID CASING SO AS TO BEAR FRICTIONALLY THEREAGAINST AND A REVERSELY BENT PORTION INTEGRAL WITH SAID ARMS AND EXTENDING GENERALLY DIAMETRICALLY ACROSS THE INTERIOR OF SAID CASING; (C) A FIXED PIN CONTACT NORMALLY PROJECTING AXIALLY INTO FRICTIONAL ENGAGEMENT WITH SAID REVERSELY BENT PORTION TO ESTABLISH AN ELECTRICAL PATH THEREBETWEEN; (D) A COIL SPRING ACTING UPON SAID MOVABLE CONTACT TO CAUSE SEPARATION THEREOF FROM SAID FIXED CONTACT, AND (E) A TEMPERATURE SENSITIVE ELEMENT NORMALLY RESISTING MOVEMENT OF SAID MOVABLE CONTACT UNTIL SAID TEMPERTURE SENSITIVE ELEMENT HAS BEEN SOFTENED BY HEAT. 